1. Field of the Invention
This invention pertains to transfer cases in vehicles having multiple drive modes, and more particularly pertains to transfer cases having an actuator for shifting between drive modes.
2. Description of the Related Art
Four-wheel drive vehicles generally incorporate some manner of transfer case by which torque from a single output shaft from a power source is transferred to two output shafts for driving separate axles of a vehicle. In a standard configuration of a four-wheel drive vehicle the rear wheels constantly receive torque from the power source and the front wheels receive torque selectively, for example, "on-demand," when the rear wheels slip, or "part-time," when an operator shifts the transfer case to four-wheel drive mode.
Various transfer case designs address different vehicle operating conditions and requirements. One style of transfer case incorporates an input shaft having a splined end, and a fixedly attached sun gear that cooperates with a planetary gear assembly. A shifting mechanism couples one output shaft to the planetary gear assembly or directly to the input shaft to provide different ranges of operation (e.g., low range, high range). That one output shaft may then be selectively coupled to a second output shaft to transfer torque thereto.
One method of transferring torque between output shafts uses an overrunning roller clutch. Such a transfer case is shown in Fogelberg U.S. Pat. No. 4,124,085. Basic roller clutch design is known in the art.
The basic design of a roller clutch, without the overrunning feature, has concentric races with rollers (preferably needle rollers) located between the races. One of the races (typically the inner race) has a plurality of cam surfaces for engaging the rollers and thus is designated the driving race or driving member. When either race rotates faster than the other, the rollers jam (i.e., lock) the cams and the driven race, thus engaging the clutch to transfer torque. Each roller can lock in one of two positions: a "retarded" position, that is at a trailing edge of a cam surface or an "advanced" position located at a leading edge of each cam surface.
In an overrunning clutch, the rollers are biased into a retarded position, that is biased opposite the direction of rotation, by a drag member. This allows the driven member to overrun the driving member without engaging the rollers bearings on the cams. However, when the driving member begins to overrun the driven member, the rollers quickly engage and torque from the driving member is transfered to the driven member.
Drag members add inefficiency to the roller clutch assembly because the drag forces reduce fuel efficiency and generate substantial heat, particularly during high speed operation. That heat requires the use of specific materials that can withstand the friction and high temperatures for long periods of time. Typically, such materials are more expensive than less wear-resistant and heat-resistant materials.
Sometimes the driven member can overrun sufficiently to overcome the force that retards the rollers. The rollers then advance, that is progress forward in the same direction of rotation as the races. If the rollers advance sufficiently, they can engage the cams so that the driven member actually drives the other (defeating the overrun feature). If this occurs, the clutch can lock up and remain in the engaged configuration even when the driven member is not trying to overrun the driving member. This condition causes an awkward driving experience.